Combined effects of oxalic acid sublimation and brood breaks on Varroa mite (Varroa destructor) and deformed wing virus levels in newly established honey bee (Apis mellifera) colonies

Annual losses of honey bee colonies are high in the United States, with losses often attributed to the effects of the ectoparasitic mite, Varroa destructor, and its associated viruses (e.g., Deformed Wing Virus, DWV). In the spring, beekeepers rely on package bees to replace lost colonies. However, these packages often come with high levels of mites and are treated with miticides upon installation. Oxalic acid (OA) is a popular miticide used in spring, but little is known about its long-term efficacy for mite control. On the other hand, natural behavior that helps with mite control is swarming, which results in a break in the production of brood in a colony. In this study, we investigated the effect of spring OA sublimation and summer swarming on colony health and productivity. We found that OA sublimation is an effective method to increase mite drop from the colony in the first three days after treatment. However, mite populations recovered quickly leading to high mite levels in the fall for colonies that did not swarm during the summer. OA applications had a marginally significant negative effect on honey yield. Mid-season swarming was correlated with lower levels of Varroa mites suggesting that implementing a brood break in the middle of the summer could play an important role in reducing mite populations for colonies originating from packages. However, the combination of OA applications to packages in the spring and summer brood break had a significant effect on reducing mite levels for colonies entering the winter. We conclude that treating packages with OA in the spring in combination with summer mite treatments are powerful tools for beekeepers to keep colonies with lower pest loads entering the winter.